1. Technical Field
The present invention relates to electronic compasses, and more specifically to a circuit and method for compensating for excess vehicle magnetic fields in an electronic compass.
2. Discussion
The present invention is related to and is an improvement of U.S. Pat. No. 4,622,843 to Hormel issued Nov. 18, 1986 entitled "Simplified Calibration Technique and Auto Ranging Circuit for an Electronic Compass Control Circuit". The present invention is also related to U.S. Pat. No. 4,807,462 issued Feb. 28, 1989, to Rafi A. Al-Attar and entitled "Method for Performing Automatic Calibrations in an Electronic Compass." These patents are hereby incorporated by reference. The present invention is related to and combinable with the commonly assigned patent application "Shifting System and Method for an Electronic Compass," Ser. No. 07/815,267.This application is hereby incorporated by reference.
Normally, electronic compass systems employ a magnetic flux-gate sensor. The operation of the flux-gate sensor is well documented. See, Hisatsugu Itoh, "Magnetic Yield Sensor and Its Application to Automobiles", SAE Paper No. 800123, pages 83-90, February, 1980; and Doug Garner, "A Magnetic Heading Reference for the Electro/Fluidic Autopilot," Sport Aviation, Part I, pages 19-26, November, 1981 and Part II, pages 20-32, 51, December, 1981. These documents are hereby incorporated by reference.
Electronic compasses are capable of processing ordinary vehicle and earth magnetic fields. In the '843 patent to Hormel, the output of an integrator is summed with the output of a ranging circuit and the sum is fed back to the flux-gate sensor. The feedback current reduces the output voltage of the flux-gate sensor, and consequently the integrator, until feedback current is equal in magnitude but opposite in direction to the current in the sense coils produced by the earth and vehicle magnetic field.
However, the primary purpose of the ranging circuit is to bring the output of the integrator into a range that the analog-to-digital IC can resolve clearly. Therefore, the ranging circuit is not capable of compensating for abnormally large vehicle magnetic fields, making the geometric method of calibration found in the '462 and '843 patents difficult to perform. The feedback current is limited by the resolving power of the analog-to-digital converter, where resolving power is defined as the ability of the analog-to-digital converter to partition an analog signal into distinct voltage ranges and assign a different value to each range. The feedback current is also limited by the size of the feedback resistors, and the inherent feedback limitations of the ranging circuit.